Heretofore, as a tire capable of safely running over a certain distance without losing its load supporting ability even at a condition that an inner pressure of the tire is dropped due to puncture or the like or a so-called run-flat tire, there is known a run-flat tire of a side reinforcing type wherein a side reinforcing rubber layer having a crescent shape in section is disposed in a sidewall portion of the tire to improve a stiffness of the sidewall portion. For example, JP-A-H11-310019 discloses a run-flat tire in which a side reinforcing layer made of a rubber composition alone or a composite of a rubber composition and fibers or the like is disposed for improving the stiffness of the sidewall portion. However, during the running at the condition that the inner pressure of the tire is dropped or so-called run-flat running, as the deformation of the sidewall portion of the tire becomes large, the deformation of the side reinforcing rubber layer becomes also large, and as a result the heat generation of the side reinforcing rubber layer proceeds, and in some cases the temperature of the tire may reach 200° C. or higher, and under such a condition the rubber component itself in the side reinforcing rubber layer may be cut or a crosslinked portion between rubber components formed by vulcanization may be cut. In this case, there is a risk that the elastic modulus of the side reinforcing rubber layer is lowered and the bending of the tire further becomes large to promote the heat generation of the side reinforcing rubber layer and eventually the side reinforcing rubber layer exceeds its breaking limit to cause troubles of the tire relatively early.
As a means for delaying a time of causing such troubles, there are known a method wherein the elastic modulus of the side reinforcing rubber layer is increased by changing a formulation of a rubber composition applied to the side reinforcing rubber layer or a bead filler of the tire, and a method wherein a loss tangent (tan δ) of the side reinforcing rubber layer is lowered to suppress the heat generation of the side reinforcing rubber layer itself.
For example, WO 02/02356 proposes that a rubber composition comprising various modified conjugated diene-aromatic vinyl copolymers, a heat resistance improver and the like is applied to the side reinforcing layer and the bead filler. Also, JP-A-2004-74960 proposes that a rubber composition comprising a specific conjugated diene-based polymer and a phenolic resin is used in the side reinforcing layer and the bead filler. They are to increase the elastic modulus of the rubber composition used in the side reinforcing layer and the bead filler but also suppress the lowering of the elastic modulus at higher temperatures, and attain a large improvement of a run-flat durability.
However, the conventional approach from the formulation of the rubber composition applied to the side reinforcing rubber layer is critical and it is present status to increase the amounts of the side reinforcing rubber layer and the bead filler for ensuring at least certain durable distance during the run-flat running. However, when the amount of the side reinforcing rubber layer or the bead filler is increased, the longitudinal spring of the tire during usual running is increased to cause problems such as deterioration of ride comfort, deterioration of noise level, the increase of tire weight and the like in the usual running.
On the other hand, cellulose-based fibers such as rayon fibers or the like have been used as a reinforcing material for various rubber goods commencing with a reinforcing cord for the tire because they have a high elastic modulus at room temperature and are high in the adhesion to rubber. Also, the cellulose-based fibers are high in the Young's modulus at room and higher temperatures as compared with a polyester such as PET or the like and have a high heat dimensional stability as the heat shrinkage at 177° C. is 0.65 to 1.0%. Therefore, the cellulose-based fibers have also been used as a reinforcing cord for a carcass in the run-flat tire of the side reinforcing type.
However, the conventional run-flat tire of the side reinforcing type using the cellulose-based fiber cords such as rayon or the like as the reinforcing cord for the carcass is large in the bending of the tire during run-flat running because the elastic modulus of the cellulose-based fiber is not sufficiently high, and also as the temperature of the tire becomes higher during the run-flat running, the stiffness of the carcass ply is lowered to further enlarge the bending of the tire. On the contrary, there is proposed a tire wherein a polyketone fiber cords are used as the reinforcing cord for the carcass, but there is a fear of deteriorating the shape of the tire manufactured because the polyketone fiber cord is shrunk by heating in the production of the tire (see JP-A-2000-264012).